One recent approach for the manufacture of CMOS devices is to effect ion implantation on a substrate through a KrF resist film as a mask in order to form p- and n-wells in the substrate. As the resist pattern is reduced in size, a replacement by a ArF resist film is in progress, and the ArF immersion lithography is proposed for further miniaturization. In order to carry out ion implantation, the substrate surface must be exposed through space portions in the resist film pattern. If a bottom antireflective coating (BARC) layer is present beneath the resist film, ions can be trapped by the BARC layer. However, if the photoresist film is patterned in the absence of the BARC layer, then standing waves generate due to substrate reflection, whereby the resist pattern after development has substantially ridged sidewalls. For the purpose of smoothening such ridges due to standing waves, it is believed effective to enhance acid diffusion by using a photoacid generator (PAG) capable of generating a low molecular weight acid which is prone to diffuse and performing PEB at higher temperature. As long as the size at which the resist film subject to ion implantation is resolved by the KrF lithography is in the range of 200 to 300 nm, it is not recognized that resolution is degraded by enhancement of acid diffusion. However, when the size at which the resist film subject to ion implantation is resolved by the ArF lithography is reduced to less than 200 nm, undesirably the enhancement of acid diffusion can cause degradation of resolution and increase the proximity bias.
The most traditional means for preventing generation of standing waves is a dyed resist material for forming a photoresist film which is absorptive in itself. The study on that means started from the novolac resist materials for i and g-line exposure. As the absorptive component which can be used in the ArF lithography, a study was made on the introduction of benzene ring into a base polymer and the use of an additive having benzene ring. However, it is impossible for the absorptive component to completely prevent standing waves. If the component is made more absorptive, standing waves are reduced, but the cross-sectional profile of a resist pattern can be tapered into a trapezoidal shape.
It is also studied to form an antireflective coating (TARC) on top of a resist film. TARC is effective for reducing standing waves, but not effective for preventing halation due to irregularities on the substrate. Ideally, the refractive index of TARC is equal to the square root of the refractive index of a photoresist film. Since a methacrylate polymer used in the ArF resist film has a relatively low refractive index of 1.7 at the wavelength 193 nm, there is available few materials having a low refractive index equal to the square root of that refractive index, 1.30.
The study was then made on developer-soluble bottom antireflective coating (DBARC, see Non-Patent Documents 1 and 2). The early study was focused on the BARC which is soluble non-isotropically in developer. This approach encountered difficulty of size control in that undercuts were formed beneath the resist pattern upon excessive progress of dissolution, whereas scum was left in space portions upon shortage of dissolution. Studied next was photosensitive BARC. In order for a material to function as BARC, the material should have an antireflective effect, and when a photoresist solution is coated thereon to form a photoresist film, the material should not dissolve in the photoresist solution and be devoid of intermixing with the photoresist film. The dissolution in photoresist solution and the intermixing can be prevented by inducing crosslinking during bake after coating of a BARC solution. If BARC is provided with a positive photosensitive function, the exposed region must dissolve in developer like the photoresist film. The crosslinked BARC film has the problem that dissolution in developer is difficult even after deprotection of acid labile groups, leaving scum in the space region.
In the case of the prior art crosslinking BARC, after resist development to form a resist pattern, the substrate is processed by dry etching through the resist pattern serving as a mask. There is a problem that the resist pattern is reduced in film thickness when the BARC film is opened. Thus a BARC having a high etching rate is required. Since the BARC film is almost unexpectable to have etch resistance, an improvement in etch resistance to enable substrate processing is assigned to the resist film. As the resist film is made thinner, the etching resistance becomes lower. An attempt to provide BARC with etching resistance to enable substrate processing encounters a dilemma that the resist pattern is more damaged when the BARC film is opened.